1. Field of the Invention
This invention relates to sulfur recovery plants of the type used to remove hydrogen sulfide from gas streams.
Removal of hydrogen sulfide from gas streams in refineries and natural gas plants has received increasing attention due to concern about atmospheric pollution. The most prevalent method for removing hydrogen sulfide is the well-known modified Claus process, which removes hydrogen sulfide by converting it to elemental sulfur. In the modified Claus process, hydrogen sulfide is partially combusted with air in a reaction furnace to form sulfur dioxide. The combustion gases are cooled in a waste heat boiler in which a portion of the uncombusted hydrogen sulfide reacts with sulfur dioxide to form elemental sulfur and water vapor. The partially converted mixture then flows to a condenser where the elemental sulfur is removed in molten form. The remaining gases are then heated and passed over a catalytic converter bed for further conversion to elemental sulfur and then again cooled to condense incremental sulfur. From one to four stages of reheat, conversion and condensing are typically used. A coalescer is usually provided to remove entrained liquids (sulfur) from the final condenser tail gas. In many cases, a tail gas cleanup unit such as the well-known SCOT unit is utilized to clean up the tail gas from the modified Claus process.
In the modified Claus unit, provisions are made to remove liquid sulfur from each vessel in which condensation of sulfur may occur. Sulfur condensation may be due to intentional design, poor operation, or equipment oversizing. Vessels in which sulfur condensation may occur include the waste heat boiler, condensers, reaction converters and the coalescer. Sulfur is generally removed from these units through sulfur piping and sulfur seal legs into a sulfur accumulator. The sulfur seal legs provide a liquid seal against the pressure established in the sulfur recovery equipment, as it is necessary to prevent toxic gas from passing into the sulfur accumulator where it might be vented to the atmosphere.
2. The Prior Art
In sulfur recovery plants, plugging of the sulfur piping with solid sulfur frequently occurs. The sulfur piping is generally steam jacketed in order to prevent formation of solid sulfur, but nevertheless such sulfur solidification frequently occurs at various points in the sulfur piping. Additionally, plugging can occur from accumulations of iron oxide, catalyst or a combination of these materials and sulfur.
The conventional manner of providing for removal of solid plugs from sulfur piping involves installation of cross-connections at each direction change in the sulfur piping. In other words, instead of an ordinary elbow in the piping, a cross-connection having four nozzles is generally utilized, and two adjacent nozzles are covered with blind flanges. In the event of a solid plug forming, the conventional procedure is to remove a blind flange from the cross-connection and break up the plug by striking it with a solid rod inserted through the blind flange opening. In many instances, union regulations require that a pipefitter remove the blind flanges, and this can produce a delay before the pipefitter is available during which poor operation, pollution, or a unit shutdown can result. Shutdown of a modified Claus sulfur recovery unit can result in the shutdown of all units producing hydrogen sulfide, and this can seriously disrupt refinery or gas plant operations.
The above-described method of removing solid plugs from the sulfur piping can also result in a serious safety hazard due to exposure of workmen to the highly toxic hydrogen sulfide gas which is prevalent throughout the plant.
U.S. Pat. Nos. 437,987 and 1,836,101 relate to cleaning of boiler tubes with steam directed through the boiler tubes. U.S. Pat. No. 3,936,892 describes a plumbing fitting including an arm joining a line, the arm having provision for connection to high pressure fluid or in the alternative to a cleanout rod. None of the prior art devices or procedures noted above provide a satisfactory solution to the problem of clearing plugs from sulfur piping in a sulfur recovery plant.